Keywords: جریان خون پلاستیکی; Arterial relaxation; Melatonin; Gestation; Nutrient restriction; Placental blood flow; SheepBK, bradykinin; EC50, concentration necessary to produce 50% of the maximal response; GMP, guanosine 3′,5′-cyclic monophosphate; NO, nitric oxide; pD2, −log (M) EC